It is known to selectively enable or disable a vehicle air bag or other occupant protection device based on the presence of an occupant in a seat. It has been proposed to place electrically conductive material in a vehicle seat to serve as an electrode for detecting the presence of an occupant in the seat. For example, U.S. Patent Application Publication No. 2009/0267622 A1, which is hereby incorporated herein by reference, describes a vehicle occupant detection system that determines the presence of an occupant based on the electrode's electrical characteristics. The electrical characteristics may be determined by coupling an excitation signal to the electrode to generate an electrode signal, and processing the electrode signal to determine an electrode signal magnitude that is indicative of an occupant presence.
The conductive material forming the electrode may behave like an antenna, whereby a noise signal may be coupled into the system such that the accuracy of determining the occupant presence is diminished. For example, a noise signal may be generated by an inverter that receives 12 Volt direct current (VDC) power from the vehicle electrical system and generates 60 Hertz (Hz)/110 Volt alternating current (VAC) power to operate a device such as a laptop computer. Noise generated by such an inverter may be characterized as comprising a 60 Hz fundamental frequency and a series of harmonic frequencies based on integer multiples of 60 Hz (e.g.—120 Hz, 240 Hz, 360 Hz, 480 Hz, . . . ). If an occupant detection system is configured to output an excitation signal having an excitation frequency of 2070 Hz, then it may be necessary to filter harmonics of the noise signal. Specifically, it may be necessary to filter 2040 Hz, the 34th harmonic of 60 Hz, and 2100 Hz, the 35th harmonic of 60 Hz, out of the electrode signal. Testing indicates that a band-pass filter centered at 2070 Hz should reduce the harmonics of the noise signal by at least 40 decibel (dB) to be effective for preventing a loss of occupant detection accuracy. It will be appreciated by those skilled in the art that a practical implementation of such a filter is by way of a digital filter, for example a 1040 tap digital band-pass filter. Such a digital filter has 1040 filter coefficients that are each multiplied by one of 1040 electrode signal values previously received by periodically sampling the electrode signal at a sampling rate. The digital filter then adds these 1040 multiplication results or terms together to calculate a filter output value. It will also be appreciated that to reliably detect a peak value of the electrode for determining the magnitude of the electrode signal, a sampling rate of ten or more times the excitation frequency is typically used for sampling such an electrode signal. Digital signal processors are known that have circular buffers to index and shift the incoming samples in preparation to multiply each sample by the appropriate filter coefficient. Such digital signal processors may also have special hardware to readily add the terms produced by the multiplication to calculate a filter output value. For the example given above, a 2070 Hz excitation frequency may need to be sampled at 20.7 kHz to assure that a peak value of the electrode signal is reliably detected. To output filter values at a rate equal to the sample rate, a processor must perform 1040 multiplies and 1040 additions 20700 times per second, or 2080 operations for each sample input value However, digital signal processors and high-speed general purpose microprocessors having such capability have an undesirably high cost when considered for use in a vehicle such as an automobile.